Auxiliary power generation systems generate power, often electrical power, for equipment to use when power from a main provider such as a utility company is out, i.e. temporarily not available, when the equipment is located in a remote area beyond the reach of a utility company's power distribution grid, such as a in a remote village, or when the equipment has unique power requirements that are not effectively satisfied by the power provided by a utility company. Because such auxiliary power generation systems often need to generate power in remote areas, many of such systems generate electrical power from the energy in a fluid such as wind, steam and running water, or from the energy in sunlight.
The systems that generate electrical energy from energy in a fluid typically include a turbine that extracts some of the fluid's energy (in the form of fluid pressure or head) to rotate a turbine shaft, and a generator that takes some of the energy in the rotating turbine shaft to move a magnet, and thus a magnetic field, across an electrically conductive material (typically a coil of copper wire) to generate a voltage in the material. Because fluid flows through the turbine to transfer energy from the fluid to the turbine's shaft, there is a rotational speed of the turbine's shaft at which the maximum amount of energy is extracted from fluid flowing through the turbine. As discussed in greater detail in conjunction with FIG. 2, this optimal speed largely depends on the pressure of the fluid entering the turbine. When electrical energy that is generated by the generator is consumed—i.e. current flows through the conductive material in the generator—the consumption opposes the rotation of the turbine's shaft, in affect acting as a brake on the shaft's rotational speed. Moreover, the magnitude of the opposing force on the turbine's shaft is directly proportional to the rate at which electrical energy is consumed—i.e. the amount of electric power consumed. Thus, the rotational speed of a turbine's shaft depends on the pressure of the fluid flowing through the turbine and the amount of electric power that is consumed.
FIG. 1 is a graph showing a typical relationship between the amount of electric energy 20, in terms of electric power, that a conventional turbine-generator combination can generate from fluid flowing through the turbine, and the fluid pressure 22 when the fluid contacts the turbine's runner. As can be seen, the amount of electric energy 20 that a turbine-generator combination can generate from a flow of fluid is directly proportional to the pressure 22 of the fluid contacting the turbine's runners. FIG. 2 is a graph showing a typical relationship between the rotational speed 24, in revolutions per minute (rpm), of a conventional turbine's shaft and the percent 26 of the total energy in the fluid flowing through the turbine that is converted into kinetic energy expressed in the rotating turbine shaft, for two different specific fluid pressures. This graph is different than the graph shown in FIG. 1, because each of the two curves 27a and 27b in this graph is limited to a specific pressure in the fluid flowing through the turbine. Curve 27a shows a typical relationship between the turbine shaft's rotational speed and the percent of the total energy in the fluid flowing through the turbine that is expressed in the rotating shaft for a fluid pressure of 40 pounds per square inch (psi). Curve 27b shows a typical relationship between the turbine shaft's rotational speed and the percent of the total energy in the fluid flowing through the turbine that is expressed in the rotating shaft for a fluid pressure of 60 psi. For each fluid pressure there is a unique curve similar to the curves 27a and 27b. More specifically, for each specific fluid pressure there is a unique optimal turbine shaft speed, 28a for the curve 27a (40 psi), and 28b for the curve 27b (60 psi), at which a maximum amount of energy is extracted from the fluid flowing through the turbine. And for each specific fluid pressure, rotating the turbine's shaft faster or slower than the optimal speed by, for example, consuming less or more, respectively, electrical power from the generator, reduces the total amount of energy that can be extracted from the fluid flowing through the turbine, and thus reduces the total amount of electric energy that the turbine-generator combination can generate.
FIG. 3 is a graph showing a typical relationship between the pressure 30 of a fluid flowing through the turbine when the fluid contacts the turbine's runner and the rotational speed 32 of the turbine's shaft that provides the maximum percentage of the fluid's total energy that is converted into kinetic energy expressed in the rotating turbine shaft. This graph, like the graph shown in FIG. 1, shows the relationship over a range of fluid pressures. Thus, this graph can be considered a collection of the unique optimal turbine shaft speeds (one of which, 28, is shown in FIG. 2) for each fluid pressure throughout the range of fluid pressures.
From FIGS. 1-3, one can see that the amount of energy transferred from the fluid to the rotation of the turbine's shaft depends on the fluid pressure and the rotational speed of the turbine's shaft. Because the rotational speed of the turbine's shaft also depends on the fluid pressure, and the amount of electric power generated, most power generation systems that include a turbine-generator combination are designed to receive a flow of fluid whose pressure remains constant, and to generate a constant amount of electric power. Furthermore, the fluid pressure and the amount of electric power generated are selected to keep the turbine's shaft rotating at the turbine's optimal speed (28 in FIG. 2) for the specific fluid pressure, and thus allow the turbine-generator combination to extract a maximum amount of energy from the fluid flowing through the turbine.
Unfortunately, in many remote areas where an auxiliary power supply uses water running in a stream or river, or wind moving through a canyon or across a plain, the fluid pressure can vary substantially over time. Consequently, the rotational speed of the turbine, and the amount of power generated, can vary over time. For example, if the fluid pressure is less than the specific pressure that a turbine is designed for, then the turbine's shaft will rotate slower than the optimal speed that transfers the most energy from the wind or running water and the generator will produce less electrical power. Thus, some of the energy in the wind or running water that could be available to generate electrical power may not be utilized. If, on the other hand, the fluid pressure is greater than the specific pressure that the turbine is designed for, then the turbine's shaft will rotate faster than the speed that transfers the most energy from the wind or running water. Although the generator would produce more electrical energy, the available excess power would not oppose the rotation of the turbine's shaft because the excess power would likely not be consumed. Thus the shaft would be allowed to rotate faster than desired. If this overspeed condition is sustained, it may damage the turbine and/or generator equipment such as the bearings for the rotating shaft(s).